(1) Field of the Invention
The present invention relates generally to radio frequency (RF) power amplifiers and, more particularly, to an RF power amplifier having a first-stage (driver) that also provides a DC current reference for an output stage.
(2) Description of the Prior Art
Amplifier architectures generally used in RF power amplifier output and driver (intermediate) stages intended for application in battery powered communications equipment often utilize DC bias circuitry. Such circuitry is used to establish a DC (current or voltage) reference for the active (amplifying) portion of the RF power amplifier. Because the DC current in the DC bias reference circuitry is not used by the RF power amplifier to amplify the incoming RF signal, this portion of the DC power consumed is inherently dissipated, thereby reducing the overall efficiency of RF power amplifiers, i.e. Class A and Class A/B amplifiers, used in those applications.
Class A amplifiers implemented in Gallium Arsenide (GaAs) Heterojunction Bipolar Transistor (HBT) processes, often utilize a reference current that is passed through a reference diode. The DC current that passes through the reference diode is generally "mirrored" to bias the output driver stage of the amplifier. Approximately one-eighth of the DC power consumed by the amplifier is often dissipated in the DC reference bias circuitry in order to mitigate variations in DC beta. Therefore, the power consumed in the bias stage is not useable to linearize devices in the active stage of the amplifier. This power loss is not desirable for small signal amplifier applications or where driver stages are used in power amplifiers.
Further, many multistage RF amplifiers utilize AC feedback from the output of the amplifier to the input of the amplifier to establish the input/output impedance of the amplifier and to linearize the transfer function of the amplifier. Amplifiers that utilize feedback from the output to the input inherently reduce desired reverse isolation and degrade the overall noise figure of the amplifier. In applications where the amplifier is used as a driver to an RF power amplifier, any such reduction in reverse isolation of the driver amplifier will make the cascade of amplifiers more prone to instability. Additionally, RF power amplifiers often exhibit large variations in input impedance due to changes in the load they are driving, e.g., antenna in a handheld wireless device. Driver amplifiers that do not provide adequate reverse isolation will reflect any such variation in input impedance from the power amplifier back to their input. Because the device preceding the driver amplifier is often a filter, variations in the driver amplifier input impedance will adversely effect the frequency response of the filter.
When associated with GaAs HBT amplifiers, DC bias circuitry must be designed to prevent thermal runaway that can damage the device. However, DC bias circuits in GaAs HBT amplifiers are prone to thermal runaway when current mirroring is implemented without DC feedback. Generally, thermal runaway can occur when biasing without negative DC feedback because of the negative temperature coefficient of the base-emitter voltage and the negative temperature coefficient of the DC beta of the device. In view of the above, DC feedback in the emitter (or base) of the active device and bias circuit is required to avoid damage to the device.
Thus, there remains a need for a new and improved RF amplifier architecture that overcomes the above described problems associated RF power amplifiers using DC bias circuitry to establish a DC reference for the active portion of the amplifier.